1. Field of Invention
The present invention generally relates to semiconductor processing. More particularly, the present invention relates to forming low dielectric constant structures including air gaps during semiconductor device fabrication.
2. Discussion of the Related Art
As device dimensions continue to shrink, intra-level line-to-line capacitance increasingly dominates over inter-level capacitance, and thus, it becomes increasingly important to implement low dielectric constant (k) schemes between tightly spaced metal lines. Reducing the capacitance between the interconnect metal lines on an integrated circuit chip will enhance the speed of the device and reduce extraneous signal energy (cross-talk) from traversing from one metal line to another.
Although air gaps formed between metal lines during dielectric deposition have been found to reduce the intralevel capacitance, they present a number of integration and reliability issues that must be addressed. In particular, the use of air gaps presents the issue of physical compromise of the air gap during subsequent processing, such as in chemical mechanical polish (CMP) or via formation. If air gaps extend above the metal lines, then the air gaps will be opened during subsequent CMP. Even if the CMP does not remove enough dielectric to reach an air gap directly, the normal and shear forces during CMP may create enough stress to compromise the interconnect structure.
Furthermore, a problem with depositions over metal lines formed with non-uniform line spacings is that an air gap formed between wider-spaced lines during the same deposition will be positioned higher than between narrower-spaced lines. There is also the possibility of forming a low quality “seam” that continues through the deposition even after the air gap has pinched off, which can lead to structural compromise.
Therefore, what is needed is a method for accurate control of the air gap height relative to the height of features above a substrate to ensure air gap integrity during subsequent processing such as CMP and via formation.